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.TH SOCKATMARK 3XNET "Oct 1, 2003"
.SH NAME
sockatmark \- determine whether a socket is at the out-of-band mark
.SH SYNOPSIS
.LP
.nf
\fBcc\fR [ \fIflag\fR ... ] \fIfile\fR ... \fB-lxnet\fR [ \fIlibrary\fR ... ]
#include <sys/socket.h>

\fBint\fR \fBsockatmark\fR(\fBint\fR \fIs\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBsockatmark()\fR function determines whether the socket specified by the
descriptor \fIs\fR is at the out-of-band data mark. If the protocol for the
socket supports out-of-band data by marking the stream with an out-of-band data
mark, the \fBsockatmark()\fR function returns 1 when all data preceding the
mark has been read and the out-of-band data mark is the first element in the
receive queue. The \fBsockatmark()\fR function does not remove the mark from
the stream.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, the \fBsockatmark()\fR function returns a value
indicating whether the socket is at an out-of-band data mark. If the protocol
has marked the data stream and all data preceding the mark has been read, the
return value is 1. If there is no mark, or if data precedes the mark in the
receive queue, the \fBsockatmark()\fR function returns 0. Otherwise, it returns
\(mi1 and sets \fBerrno\fR to indicate the error.
.SH ERRORS
.sp
.LP
The \fBsockatmark()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 10n
The \fIs\fR argument is not a valid file descriptor.
.RE

.sp
.ne 2
.na
\fB\fBENOTTY\fR\fR
.ad
.RS 10n
The \fIs\fR argument does not specify a descriptor for a socket.
.RE

.SH USAGE
.sp
.LP
The use of this function between receive operations allows an application to
determine which received data precedes the out-of-band data and which follows
the out-of-band data.
.sp
.LP
There is an inherent race condition in the use of this function. On an empty
receive queue, the current read of the location might well be at the "mark",
but the system has no way of knowing that the next data segment that will
arrive from the network will carry the mark, and \fBsockatmark()\fR will return
false, and the next read operation will silently consume the mark.
.sp
.LP
Hence, this function can only be used reliably when the application already
knows that the out-of-band data has been seen by the system or that it is known
that there is data waiting to be read at the socket, either by \fBSIGURG\fR or
\fBselect\fR(3C).
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	Safe
.TE

.SH SEE ALSO
.sp
.LP
.BR select (3C),
.BR recv (3XNET),
.BR recvmsg (3XNET),
.BR attributes (7),
.BR standards (7)
